In such autonomous units, there are elements or accessories which must be actuated by onboard means in the unit itself, to be used for example in flight. Thus, for a rocket, jacks direct the jet motor or motors to guide the unit.
The energy necessary for the jacks is usually furnished by a high pressure hydraulic pump which is driven by a gas or electric motor, the whole forming on onboard group in the rocket.
Before autonomous working, numerous tests must be made. In the prior art, the energy necessary to control the various elements to be tested is delivered by offboard energy sources, gas under pressure, or electric current. These energy sources were used to drive the onboard group while avoiding using the onboard energy sources, both not to increase the onboard weight and because these onboard sources may not yet be available.
The drawback of this technique is that the onboard pumping group is expensive, heavy, and of poor reliability.